Some Aspects of Reproductive Biology of the Gametophyte Generation of Homosporous Ferns

Abstract

In the homosporous ferns, the ‘haploid’ gametophyte phase (generation) is exosporic, small, short -lived and, more importantly, is completely independent of the ‘diploid’ sporophyte generation. Hence, it is as much subject to divergence and natural selection as the dominant sporophyte generation. Further, the potentially homothallic nature of their prothalli has triggered inquiry to reveal adaptations of the gametophyte generation that tend to determine the relative probabilities of inter- and intragametophytic mating as against the presumptive notion of frequent/habitual gametophytic self-fertilization in the natural populations of homosporous ferns that leads to total homozygosity, reducing a population’s evolutionary flexibility. Reproductive biology of the gametophyte generation of about fifty species of leptosporangiate ferns from the Himalaya has been examined here in respect of (l) form and sexuality, (2) sequence, location and duration of gametangia on the eventually monoecious prothalli, (3) sexual structure of cultured gametophyte populations, and (4) the ability of isolated gametophytes to become bisexual and undergo self-fertilization (= isolate potential), besides correlations, if any, among gametophyte generation time, polyploidy, habit and habitat. The importance of ascertaining the ontogenic sequence of gametangia on meristic prothalli together-with duration of the formation of antheridia during the so-called “hermaphroditic” phase is stressed. Three parameters of the laboratory-raised gametophyte populations, namely, population index, fecundity index, and mating index, as proposed, may be useful to quantify the data for attempting comparative assessments of mating system both within and between species. Contrary to the general belief of predominant intragametophytic selfing amongst the homosporous ferns, the observations reveal a variety of strategies which may promote, disproportionately favour, or even ensure intergametophytic mating, and that the life cycle of homosporous ferns cannot be stereotyped.